Under certain conditions and after a while, equilibrium is established in both homogeneous and heterogeneous systems. By observing number of phases and components of heterogeneous equilibrium we talk about phase equilibriums. In this thesis equilibrium at solid-liquid, liquid-gaseous and solid-gaseous phase boundary is described. Other phase transition boundaries, the phase changes and phase diagrams of water and mixture of water and alcohol are also described. In addition, aplication and examples in real life are also described.